In the design of new structures and the testing of existing structures it is desirable to know the distribution of stresses within the structure.
In the design of new structures weight and cost of the structure may be minimized by designing the structure so that the stresses are uniformly distributed throughout the structure. Stress concentrations caused by faulty design often lead to failure of the structure or to a shortened useful life. In an existing structure such as an airplane or steel bridge, the stresses can often indicate incipient and hidden flaws or cracks in the structure which may lead to eventual failure. One method which has been developed for determining the distribution of stresses within a structure involves employing the thermoelastic effect.
When a given volume of gas is compressed, its temperature increases. When the gas is expanded, its temperature decreases. In a similar way, solids which are under compression or tension undergo minute changes in volume which result in small but measurable changes in temperature. Systems employing the thermoelastic principle to determine the distribution of stresses in a structure employ infrared (I.R.) cameras to detect the patterns of stress on the structures. However, the temperature change caused by applying stress to a material is very small. At room temperature the application of a stress of about 150 psi causes about 0.001 degree Kelvin temperature change in steel. A typical D.C. or steady state infrared camera is unable to see temperature changes of this magnitude. Because of the statistical nature of the way infrared radiation or the photon flux is emitted from a surface the I.R. detectors used in infrared cameras are quite noisy and hide the temperature change caused by the applied stress. When the loads are applied periodically the temperature changes corresponding to the stresses in the material become periodic in nature. Given that the temperature changes which relate to the stress in the test object are periodic it is then possible to use various signal processing techniques to reduce the signal noise and detect the stress-related temperature changes.
One type of known camera is an analog slow scan camera. This is a raster scanning signal detector camera which completely processes the I.R. signal for each scanned point on an object before moving on to the next point. It utilizes a lock-in amplifier and averaging of the sampled lock-in output to reduce signal noise. Likewise, digital slow scan cameras are known where the output of the I.R. detector is digitally sampled. Signal extraction is performed with a digital lock-in operation, and noise is further rejected by the use of a band limiting low pass filter. The slow scan cameras can achieve temperature sensitivities of 0.001 degrees K but generally require long periods of time on the order of an hour or more for a single temperature profile over a scanned object.
Another known system is the video fast scan camera. This system is based on a single detector and a fast scan mirror set, and provides its output in video format. Every point in the scan is sampled at the video frame rate, typically at 30 times per second. This results in a very short sample time for each point on the object which results in a noisy signal.
The video fast scan camera rapidly scans the object but, because of the noise problems associated with the short sample time, many scans are required to detect the temperature profile of the specimen. Hence, the overall time for obtaining a stress profile thermal image is not appreciably enhanced over a slow scan camera. The nature of the video frame rate requires that the specimen be loaded in exact synchronization with the frame rate. This synchronization requirement makes video scan systems difficult to employ. Array fast scan cameras are known which can rapidly produce a thermal image of a specimen. However these array cameras are not designed or optimized to detect the very small temperature changes produced by the stress profiles within the object.
Due to the slow speed of the thermoelastic effect imaging systems hithertofore available, the application of the thermoelastic technique for determining stresses in materials has been limited to applications where the time and expense of employing existing equipment could be justified.
What is needed is a thermoelastic detection system which can rapidly image an object and so cost effectively determine the stresses therein.